Salmonella enterica serovar Typhimurium is worldwide one of the main causes of bacterial food-borne diseases in humans. Recent reports warn that a growing number of S. Typhimurium strains are becoming multi-resistant to various antibiotics. The quorum sensing systems of S. Typhimurium regulate the expression of a number of genes which are putatively involved in virulence and biofilm formation. This suggests that interference with quorum sensing could be an alternative strategy for the treatment of Salmonella infections and biofilms. Brominated furanones, which are secondary metabolites of the Australian red macro-alga Delisea pulchra, are able to inhibit quorum sensing and biofilm formation in various bacteria without influencing the growth. However, nothing is known about their activity on Salmonella.
In this study, we chemically synthesized a series of brominated furanones with variations in the length of the 3-alkyl chain and in the bromination pattern of the ring structure. Subsequently we screened these molecules for their biological activity on S. Typhimurium. We tested the influence of the molecules on luminescence-based reporters for the activity of the two quorum sensing systems of S. Typhimurium, namely the acyl-homoserine lactone receptor SdiA and the LuxS/AI-2-system. Addition of the brominated furanones reduced the luminescence produced by both reporters in a concentration-dependent way and a structure-activity-relationship of the furanones was determined. Remarkably, the furanones did also quench the luminescence of sdiA and luxS mutants. This indicates that the observed effect of the furanones on the quorum sensing reporter systems is not only dependent on the presence of SdiA and LuxS. The origin of the reduced luminescence remains to be elucidated. The nature of the quorum sensing regulated genes in S. Typhimurium suggests a role for quorum sensing in the Salmonella biofilm formation. This prompted us to investigate the influence of the furanones on Salmonella biofilms. The molecules were shown to inhibit biofilm formation in an early stage at concentrations that do not influence the growth of planktonic Salmonella cells. However, when added at later stages during the biofilm formation process, the molecules were less effective. Salmonella biofilms show a strongly increased resistance to antibiotics. Interestingly, we found that a furanone treatment makes Salmonella biofilms more sensitive to degradation by tetracycline.